A phototransistor is generally formed utilizing a process of manufacturing semiconductor devices, on a monocrystalline semiconductor substrate, and a light signal processing circuit including normal transistors and so forth is also formed on the semiconductor substrate at the same time in addition to the phototransistor. Accordingly, it is possible to sense even a very small quantity of light with a high sensitivity by an amplification function of the signal processing circuit.
The step of forming a semiconductor junction is a particularly important step in a manufacturing process of a phototransistor. The semiconductor junction can be formed by doping conductivity type determining impurities from a surface of a monocrystalline semiconductor substrate into the junction. A thermal diffusion method is generally used as a specific method of doping the impurities into the junction.
However, according to the thermal diffusion method, a semiconductor junction plane is formed in a part at a relatively large depth from the surface of the monocrystalline semiconductor substrate, and that depth lowers the photosensitivity of the phototransistor for light having a shorter wavelength which is absorbed mainly in a part at a smaller depth. Therefore, in recent years, consideration has been given to a method of forming a semiconductor junction by depositing an amorphous semiconductor thin film on a monocrystalline semiconductor substrate as a method by which it is possible to control the position of a semiconductor junction plane on the order of .ANG.. However, a semiconductor junction between a monocrystalline semiconductor substrate and an amorphous semiconductor film becomes a so-called heterojunction. The interface characteristics of a heterjunction are generally inferior as compared with the interface characteristics of a conventional homo-junction formed inside a monocrystalline semiconductor substrate by the thermal diffusion method. The reason for this mainly exists in interface energy levels of the heterojunction, which are due to a number of localized energy levels existing in an energy bandgap of an amorphous semiconductor film.
When a number of interface energy levels exist, even if many charge carriers are generated in the vicinity of the heterojunction interface by light irradiation, the interface energy levels function as a recombination center of the carriers and cause part of the carrier to disappear. Such disappearance of carriers lowers the photosensitivity of a phototransistor.